It honestly sounds like some application/driver installed on the Guest OS is not being handled properly. When you boot into safe mode you are essentially stripping the VM down to the basic OS. I'd recommend checking Event Viewer to see if there is an Application event that might lead in a better direction.
Otherwise, you can try Startup management in task manager to stop a few services from startup and see if you can narrow down the appliaction/service.
On a side note, was anything recently updated/patched/changed on these virtual machines that could be a common denominator?
